Voice coil motors, or more shortly VCMs, are widely used in many applications. They are substantially composed of a coil immersed in a magnetic field generated by a permanent magnet. By forcing through the coil a certain current, forces that displace the coil are generated. This displacement of the coil may be controlled accurately.
Besides VCMs, there are other electromechanical actuators that work by exploiting this principle, such as audio loudspeakers, electro-locking systems and the like. Because of the importance of VCMs, hereinafter reference will be made only to these kinds of motors, but the same observations hold for any voice-coil type of actuator.
VCMs are used for displacing the arm that carries the read/write head(s) from a working position overhanging a spinning disk to a safe parking position on a ramp (ramp unloading), and vice-versa (ramp loading).
The ramp unloading operation of the read/write operations is essential for preventing possible damage to the disk or to the head(s) because during transportation, the suspension arm is subject to vibrations and the heads could hit the surface of the disk damaging it and/or themselves. The opposite operation is the ramp loading and it is carried out each time the hard disk is enabled for reading from or writing data on it.
It is important that the speed of rotation of the mechanical arm be controlled during these operations for preventing possible damage to the heads when the arm reaches the run stop at the end of the parking ramp, or when the heads are brought to and held over selected tracks of the spinning disk.
Ramp loading and ramp unloading operations are controlled by a control circuit for the speed of the mechanical arm, moved by a VCM. According to a control technique for the speed of a VCM, the instantaneous speed of the mechanical arm may be measured using optical encoders, as disclosed in U.S. Pat. No. 5,455,723.
Notably, the speed of the mechanical arm may also be measured by sensing the back electromotive force (BEMF) induced in the motor coil by the motion of the arm. The back electromotive force induced by the motion of the arm is proportional to the speed of the arm.
A method and a circuit for measuring a back electromotive force are disclosed in U.S. Pat. No. 6,788,490, which is assigned to the current assignee of the present invention and is incorporated herein by reference in its entirety. The circuit generates an estimated value of the BEMF as a function of the current flowing through a coil of a magnetic actuator, measured on a current sensing resistor.
This technique is hardly appropriate for sensing the back electromotive force in VCMs controlled in a PWM or PSM mode. This is because the signal/noise ratio is generally too low for ensuring a sufficiently precise control of the speed.
The back electromotive force may be reliably measured by sensing the voltage on the nodes of the coil of the motor when the motor is in a tristate state. This is according to a technique disclosed in U.S. Pat. No. 6,542,324, which is assigned to the current assignee of the present invention and is incorporated herein by reference in its entirety.
When operating in a discontinuous mode, the stage that drives the motor alternates conduction intervals, during which the motor is connected to the power supply rail, to off intervals, during which the motor is in a high impedance state, i.e., tristated. TON is the duration of a conduction phase, and TOFF is the duration of an off phase in which the motor is tristated. For a time TOFF no current is flowing and the voltage drop on the nodes of the motor is equal to the back electromotive force (BEMF). An operational amplifier senses the back electromotive force induced in the coil of the VCM by sensing the voltage on its nodes when the switches of the driving stage are open.
A drawback of this technique is in the need to wait for a minimum time, TOFFMIN, before the voltage on the nodes of the motor approximates with sufficient accuracy the induced back electromotive force BEMF. Tests carried out on a real VCM demonstrated that, starting from the instant in which the motor is tristated, there is a transient component of the voltage on the nodes of the motor that alters the BEMF sensing.
This effect may be clearly noticed from the graphs of FIG. 2 that show sensed waveforms of the currents and of the voltages on the positive and negative nodes during the time intervals TON and TOFF. This is in two different driving conditions of the VCM. These diagrams were obtained by blocking the moving arm of the VCM for highlighting such a voltage disturbance. Thus, the voltage sensed on the coil decays exponentially and is the voltage disturbance.
When the current transient finishes, there persists a transient of a voltage disturbance yet to decay completely, highlighted by the dashed circle 9. Such a persistent disturbance typically lasts several tenths of a microsecond, and its duration depends upon the current that has been flowing in the coil during the precedent conduction phase. This can significantly fault the sensing of the BEMF. When the microprocessor μP of FIG. 1 generates a pulse 8 for activating the analog-to-digital converter ADC, the latter samples the voltage on the coil.
To sense correctly the BEMF induced in the coil of the VCM, it is necessary that the motor be tristated for a minimum time, TSAMPLE, long enough to let the voltage transient finish. This phenomenon limits the frequency of the control signals of the switches of the driving stage, and as a consequence, the precision of control of the speed of the motor. Moreover, this frequency typically ranges between 1 kHz and 3 kHz. Therefore, noise at audible frequencies is generated.